Damned DOM (1)

When I wanted to react to any changes to a input textbox immediately, my first instrinct was to use onChange. onChange, however, is called when the input loses focus. onKeyPress then? Isn’t called on backspaces. onKeyDown, maybe? It does get called, but the effect of the keystroke isn’t yet applied, for the return value determines whether it that is done in the first place. (Same story for onKeyPress by the way.) onKeyUp does work a bit, except if someone is holding down a single key, for a while.

The solution: hook onKeyUp and use setTimeout with a timeout of 0. Yugh. I hate DOM.

Big Fat Disclaimer: I actually tested this only on one browser.

Django annoyances: no reverse select_related


for page in Page.objects.all():
  print page.title
  for comment in page.comments.all():
    print comment

There will be a single query to fetch all pages, but there will be for every page another query to fetch its comments. Luckily, Django has got a nice trick up its sleave: select_related. Would I use instead of Page.objects.all(), Page.objects.select_related('comments').all() then Django will use a single joined query to prefetch comments for each page.

However, Django’s select_related only supports forward one-to-many references. No many-to-many; certainly no reverce many-to-many; no reverse one-to-many and no, not even reverse one-to-one (yet). A developer claims it’s impossible (which is bullshit), another asks for patches, which means he doesn’t care doing it himself.

It’s quite easy to manually code around the missing reverse select_related, but it takes too many ugly lines compared to the single word it could’ve been.

Stupid PHP (1) (Strings are faster than Arrays)

When I slowly build a big string out of little bits, the worst thing to do in most languages is to just use string concatenation:

for(something) {
 str .= little_bit;

Why? Everytime a little bit is added to str, there must be a new string allocated big enough to contain str and the new little bit. Then the actual str must be copied in. In most languages there are constructs to efficiently build strings like this instead of concatenating. StringBuffer in C#. StringIO in Python.

But no, PHP has to be stupid. There is no nice construct and you’ll end up using concatenation. So, I thought to be smart and make use of PHP array’s and implode. Arrays are here for having elements added and removed all the time so they are properly buffered and should be great at having lots of small elements added. And when I want to pack it all into one big string, I can use PHP’s builtin implode function.

I wanted to try it out and created two scripts: a.php concats a little (10byte) string one million times and b.php appends it to an array and then implodes it. And because I’m also interested in the performance of implode I got a script c.php that’s identical to b.php but doesn’t implode afterwards. These are the results:

a.php (concat) 0.320s
b.php (array append and implode) 0.814s
c.php (array append) 0.732s

Indeed, string concatenation with all its allocation and copying is actually faster than plain simple array appending. PHP is stupid.